Nowadays, extensive research is being performed in the field of wireless communications for improving wireless networks all around the world. The 3GPP Long Term Evolution (LTE) that is the last step toward the 4th generation (4G) of wireless communication systems, is focusing in improving the capacity and speed of wireless networks.
A wireless communication system includes a base station (BS) and a User Equipment (UE) consisting of a receiver for receiving radio signal from the base station in the downlink and a transmitter for transmitting radio signal to the base station in the uplink.
A radio transmission chain of a UE includes a power amplifier (PA) for amplifying the signal before it is transmitted through the channel. The power amplifier has an approximately constant gain that is defined by the ratio of output to input power of the amplifier and is measured in decibels.
However, in certain modulation schemes of the radio signal, the peak to average power ratio (PAPR) of the signal may be high. Specifically, the PAPR is a measurement of a waveform of the signal, calculated from the peak amplitude of the waveform divided by the Root Mean Square (RMS) value of the waveform.
A high PAPR might result in saturation of the power amplifier and accordingly to distortion of its output signal peaks. In turn, this distortion provokes increased channel interference and noise.
A known solution to this problem is the so-called Maximum Power Reduction (MPR). Specifically, MPR reduces the maximum average transmit power at which the transmitter of the UE is able to transmit, any time that the transmit signal has high peak to average power ratio.
In prior art, maximum power reduction can be performed by applying a fixed reduction according to mode (High-Speed Uplink Packet Access (HSUPA), High-Speed Downlink Packet Access (HSDPA) etc).
Another way for performing maximum power reduction (MPR) is according to a metric defined as the root mean square of the cube of the instantaneous transmit signal power Tx. In the 3GPP standard, this metric is called a “cubic metric”.
Several attempts have been made in order to estimate the cubic metric (CM) based on software look-up tables grouping values of parameters which yield the same CM (rounded to 0.5 dB).
Such solutions seem unworkable due to the high number of parameters involved (channel weighting coefficients, spreading factor) and also the large range of each parameter
The problem lies on the fact that, in order to avoid any excessive distortion of the power amplifier during the entire transmission, the MPR and thus the cubic metric needs to be determined prior to the start of uplink transmission, and thus when the data to transmit is available.
In the case of the 3G, the uplink transmission is divided into timeslots but is continuous. For each timeslot, the parameters which influence the cubic metric can change at a slot boundary, but remain constant during each timeslot. In particular, the above mentioned parameters include channel weighting coefficients, channel spreading factors and spreading codes (OVSF) as well as IQ mapping.
In many cases, the data to be transmitted in a given timeslot is not available until the start of that timeslot. This means that the cubic metric can not be directly measured on the signal transmission and in a suitable time to determine the maximum power reduction.
Accordingly, there is a need of computing the cubic metric before the signal transmission, and particularly without requiring too extensive data processing.